An electric power steering device generally determines an assist force based on a steering torque detected by a torque sensor to drive and control an electric motor. An abnormality of this torque sensor greatly influences steering auxiliary control. Therefore, it is desirable to establish a duplex torque sensor system such that even if an abnormality occurs on one system of the system, the same function of the system may be continued by switching to the other system.
However, the region in which detection of a torque is carried out mainly around a torsion bar, but it lacks space. Therefore, in order to build the duplex system, two systems have to be arranged at almost coaxially adjacent positions.
Meanwhile, as a conventional torque sensor, coil type torque sensors are widely used, which reflects the torque exerted on a revolving shaft in impedance change of the coil, for detecting the impedance change. However, when such two systems of coil type torque sensors are arranged in almost coaxially adjacent positions, as described above, magnetic coupling may generate mutual interfering between the two systems. As a result, a low-precise torque signal will be generated during regular use thereof.
There is a technology disclosed in Patent Document 1, as an example for reducing the influence of generated magnetic flux interference by use of two or more coil pairs to. According to this technology, mutually different magnetization frequencies of exciting current supplied to two sets of coil pairs, respectively, and a low-pass filter for blocking passage of frequencies corresponding to the difference between the magnetization frequencies is used to reduce an AM modulated noise oscillating at a beat (beat tone) frequency developed due to the magnetization frequency difference.